(647a) Solute Diffusion through Swollen Polymer Networks with Complex Structures | AIChE

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(647a) Solute Diffusion through Swollen Polymer Networks with Complex Structures

Authors 

Richbourg, N. - Presenter, University of Massachusettts
Peppas, D. N. - Presenter, University of Texas at Austin
Controlling solute transport in hydrogels is critical for numerous chemical separation applications, tissue engineering, and controlled permeation and delivery through membranes and drug delivery systems. In previous work, we have pointed out that proposed theoretical models and associated experiments tend to oversimplify the influence of hydrogel structure on solute transport by addressing only the effects of the polymer volume fraction and mesh size of the networks on solute transport. Understanding solute transport in hydrogels is important for molecular separation processes using hydrogel membranes, for controlling drug delivery from hydrogel reservoirs, and for managing cellular communication in hydrogel-based tissue engineering scaffolds. Fluorescence recovery after photobleaching (FRAP) experiments in hydrogels are an exceptionally accurate and fast, high-throughput method for characterizing solute self-diffusion within hydrogels. Additionally, the confocal microscope used for FRAP can quantify the partitioning of the solutes into the hydrogel by comparing the concentrations of solutes within the hydrogel and in the source solution. Solute diffusion and partitioning in hydrogels are generally understood to be affected by both the properties of the solute and properties of the hydrogel, but current models generalize the solute contributions to their hydrodynamic radii and the hydrogel contributions to their swollen polymer volume fraction, mesh size, and fiber radius

Our previous work investigating the diffusion of fluorescently tagged dextrans and linear poly(ethylene glycol) (PEG) in poly(vinyl alcohol) (PVA) hydrogels demonstrated that solute diffusivities in hydrogels do not scale consistently with hydrodynamic radius. In recent theoretical analysis, we argued that mesh size may be improved for solute diffusivity since it does not account for how the geometry of the swollen polymer network influences solute diffusivity. The proposed mesh radius correction for hydrogels with four, six, or eight chains converging at a junction aims to account for the limitations of using mesh size.

In our previous work, we investigated the diffusion of fluorescein, dextrans, and PEGs in eighteen PVA hydrogel formulations with varying initial polymer volume fraction and degree of polymerization between junctions. Here, we expand our focus on how hydrogel structure affects solute diffusivity and investigate solute partitioning with fluorescein and two sizes of dextrans in 73 formulations of multi-arm PEG hydrogels with simultaneous variation of four independent, synthesis-controlled structural parameters: initial polymer volume fraction, degree of polymerization between junctions, junction functionality, and frequency of chain-end defects. Full-factorial analysis of how these structural parameters affect solute diffusion and partitioning in hydrogels provides unprecedented insight into how the hydrogel structure affects solute transport. In this study, we investigate the influences of hydrogel structure on solute diffusion and partitioning in hydrogels and evaluate correlations between hydrogel swelling, solute diffusion in hydrogels, and solute partitioning in hydrogels.

Here, we reexamine these models by experimenting with a library of multi-arm poly(ethylene glycol) (PEG) hydrogels with simultaneous variations in four independent structural parameters. Standardized, high-throughput fluorescence recovery after photobleaching (FRAP) experiments in hydrogels characterize size-dependent solute diffusion and partitioning in each hydrogel formulation. Solute diffusivity dependence on junction functionality shows an influence from network geometry, experimentally validating the use of the geometry-responsive mesh radius in solute diffusivity modeling. Furthermore, the Richbourg-Peppas swollen polymer network (SPN) model accurately predicts how three of the four structural parameters affect solute diffusivity in hydrogels. Comparison with the large pore effective medium (LPEM) model showed that the SPN model better predicts solute size and hydrogel structure effects on diffusivity

Structure-based predictions of mesh size, mesh radius, and specific solute diffusivities in each hydrogel were made using the SPN model. Swollen polymer volume fractions were calculated via Equation 1. Mesh size was calculated from the swollen polymer volume fraction, structural parameters, and identity constants using Equation 2, a modification of the Canal-Peppas equation. Mesh radii were calculated from mesh sizes and junction functionalities using Equation 3. Solute diffusivities in hydrogels were calculated according to Equation 4, a modified multiscale diffusion model based on hydrogel and solute properties.

In these studies, we demonstrate that solute diffusion and partitioning in hydrogels are both linked to the hydrogel’s network structure, but they are not always correlated. Notably, the frequency of chain-end defects has a discerning effect on diffusivity and solute partitioning. A higher frequency of chain-end defects consistently increased diffusivity but shifted from decreasing to increasing partitioning with increasing solute size. Multi-arm PEG hydrogels have exceptional control of junction functionalities based on the number of arms per precursor molecule, allowing precise investigation of how junction functionality affects solute transport. The experimental results confirmed our theory that more geometrically restrictive networks reduce solute diffusivity even with equivalent mesh sizes. We recommend the use of mesh radius over mesh size in models relating hydrogel structure to solute diffusivity. FRAP and confocal-based partitioning methods overcome some of the problems associated with surface accumulation during solute transport in hydrogel studies. However, large polydisperse solutes may still create a screening effect in these studies where only the smaller solutes make it into the hydrogels.

Topics 

Transport Phenomena
Thermodynamics

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